An accumulator type fuel injection system is known which is adapted to stably supply a high-pressure fuel stored in an accumulator into each cylinder of a diesel engine, and enable the engine performance to be improved in a wide operating range thereof. However, when a fuel injection rate immediately after the starting of a fuel injection operation is excessively high even in such a fuel injection system, a sudden explosion combustion is carried out in an initial stage of combustion, and not only operating noise of the engine but also the NOx content of an exhaust gas increases.
To eliminate such inconveniences, an accumulator type fuel injection system adapted to inject a fuel at a lower injection rate in an initial stage of each fuel injection cycle has been proposed. This proposed system is provided with, for example, a low-pressure accumulator adapted to store a low-pressure fuel, a high-pressure accumulator adapted to accumulate a high-pressure fuel, a change-over valve adapted to selectively communicate the low-pressure accumulator or the high-pressure accumulator with an injector (fuel injection nozzle) and thereby switch an injection rate, and a switch valve adapted to communicate and shut off a control chamber of the injector with and from a fuel tank and thereby control the injection time.
Regarding the formation of a fuel pressure in an accumulator, there are, for example, an accumulator adapted to obtain low-pressure and high-pressure fuels by using a low-pressure pump and a high-pressure pump which are driven by an engine, and an accumulator (Japanese Patent Laid-Open No. 93936/1994) adapted to obtain a high-pressure fuel by using a high-pressure pump, and a low-pressure fuel by regulating the pressure of the high-pressure fuel introduced into a low-pressure accumulator.
In the accumulator type fuel injection system disclosed in International Patent Laid-Open No. WO98/09068, which is adapted to obtain a low-pressure fuel of a low-pressure accumulator from a high-pressure fuel of a high-pressure accumulator, for example, a fuel passage by which a change-over valve and a fuel chamber of an injector are connected together is filled with a low-pressure fuel by closing both an injection time control switch valve and an injection rate change-over valve, and the injector is maintained in a valve-closed state by supplying the low-pressure fuel to a control chamber of the injector which communicates with the fuel passage. At the arrival of the injection starting time, the switch valve is opened to discharge the low-pressure fuel in the control chamber to a fuel tank, whereby a valve of the injector is opened to carry out initial low-pressure injection (which will hereinafter be referred to as low-pressure injection). When the low-pressure injection period has elapsed, the change-over valve is opened to carry out main high-pressure injection (which will hereinafter be referred to as high-pressure injection) by injecting the high-pressure fuel in the high-pressure accumulator from a nozzle. At the arrival of the injection finishing time, the change-over valve is closed. In the low-pressure accumulator, a low-pressure fuel is obtained by regulating the pressure of the high-pressure fuel flowing from the fuel passage thereinto. In such an accumulator type fuel injection system, the fuel in the fuel passage gradually flows out when the engine is stopped, via a clearance around a plunger of a high-pressure pump and a clearance around the control chamber of the injector, and the fuel pressure in the fuel passage and low-pressure accumulator decreases to a level corresponding to that of the atmosphere.
When the cranking is done at a subsequent engine starting time, a pressurized fuel is supplied from the high-pressure pump to the high-pressure accumulator, and the injection rate switching control change-over valve and injection time control switch valve are opened and closed so as to carry out low-pressure injection and high-pressure injection. When the pressurized fuel is discharged from the high-pressure accumulator during a high-pressure injection period, the fuel pressure therein decreases correspondingly, and the formation of a fuel pressure in the high-pressure accumulator tends to be delayed immediately after the starting of the engine. Consequently, the formation of a fuel pressure in the low-pressure accumulator is delayed.
Therefore, it is difficult immediately after the starting of the engine to obtain a low-pressure fuel of a predetermined pressure exceeding a valve opening pressure of the injector. The low-pressure injection is not executed until a fuel pressure in the low-pressure accumulator has reached a predetermined level, and the fuel injection starting time is delayed, so that inconveniences, such as imperfect starting of the engine and the discharging of white smoke occur.